1. Field of the Invention
The invention relates in general to a memory cell and a method of manufacturing the same, and more particularly to a nitride read-only memory cell and a method of manufacturing the same.
2. Description of the Related Art
Memory devices capable of storing non-volatile information such as read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM) and other advanced memory devices are now widely applied in computers, communication devices and electronic products. Of the memory devices, advanced memory devices include electrically erasable programmable read-only memory (EEPROM), flash EEPROM and nitride read-only memory, and so on. The nitride read-only memory cell unit, having the features of storing two bits of data in one cell unit, higher memory density and compactability with the manufacturing process of the complementary metal oxide semiconductor (CMOS), has become a focus in the application and development of memory devices.
Normally, the elements of nitride read-only memory are disposed in two different areas, the array area and the periphery area. The array area comprises several oxide/nitride/oxide (ONO) structures and gate structures as the nitride read-only memory cell units. A barrier diffusion layer separates each of the ONO structures. According to the nitride read-only memory cell unit, the barrier diffusion layer and the spacer are etched on the edge of the array area, such that contact holes are formed for electrically connecting corresponding cells. However, along with the advance in the technology related to the memory manufacturing process, the memory elements are gradually scaled down to 0.075 nanometers or even 0.045 nanometers. Consequently, shifts or biases are more likely to occur during the process of etching, causing errors or biases to the contact holes. The current practice is to change the material of the spacer from oxide to nitride to facilitate the process of self-align contact (SAC) etching.
However, when the material of the spacer is nitride, the threshold voltage of the cell increases. The higher the threshold voltage is, the higher the working voltage of the memory device will be, not only intensifying the damage of the bottom oxide but also increasing power consumption of the memory device, and further reducing the stability of the memory device during operation.